OPINION

MORENO, J.

Proposition 83, passed by the voters in November of 2006, modified the terms by which sexually violent predators (SVP's) can be released from civil commitment under the Sexually Violent Predators Act (SVP Act or Act; Welf. Inst. Code, § 6600 et seq.). In essence, it changes the commitment from a two-year term, renewable only if the People prove to a jury beyond a reasonable doubt that the individual still meets the definition of an SVP, to an indefinite commitment from which the individual can be released if he proves by a preponderance of the evidence that he no longer is an SVP.

Defendant, who is subject to indeterminate commitment pursuant to Proposition 83, challenges the law on several constitutional grounds: that it violates the due process and equal protection clauses of the Fourteenth Amendment of the United States Constitution and that it violates the ex post facto clause, article I, section 10 of the United States Constitution. Like the Court of Appeal, we conclude that defendant's due process and ex post facto challenges are without merit. As for the equal protection challenge, we conclude that the state has not yet carried its burden of demonstrating why SVP's, but not any other ex-felons subject to civil commitment, such as mentally disordered offenders, are subject to indefinite commitment. As explained below, we remand to the trial court to permit the People the opportunity to justify the differential treatment in accord with established equal protection principles. (See In re Moye (1978) 22 Cal.3d 457 [ 149 Cal.Rptr. 491, 584 P.2d 1097].)

I. FACTUAL AND PROCEDURAL BACKGROUND

On November 8, 2004, a petition was filed to establish Richard McKee as an SVP within the meaning of the Act. The petition alleged McKee was "a person who has been convicted of a sexually violent offense against two or more victims for which he was sentenced and who has a diagnosed mental disorder that makes him a danger to the health and safety of others, in that it is likely he will engage in sexually violent predatory criminal behavior." It alleged he had been convicted of two counts of committing lewd and lascivious acts on a child under the age of 14 (Pen. Code, § 288, subd. (a)). One victim was an 11-year-old girl and the other was an eight-year-old girl. The petition requested that McKee be committed to the custody of the State Department of Mental Health (DMH) for a period of two years.

On February 16, 2007, McKee demurred to the petition on the ground that the Act, as amended on November 7, 2006, by the voters' passage of Proposition 83, was unconstitutional. The trial court overruled the demurrer.

On March 5, 2007, an amended petition was filed restating the original petition's factual allegations and requesting that McKee be committed to an indeterminate term pursuant to the amended Act. On March 12, following a five-day trial, the jury returned a verdict finding McKee was an SVP within the meaning of the Act. On March 13, the trial court issued an order committing McKee to the custody of the DMH for an indeterminate term pursuant to the Act.

McKee timely filed a notice of appeal. The court rejected McKee's claims that the indeterminate commitment instituted by Proposition 83 violated federal or state due process, ex post facto or equal protection provisions. The court also rejected McKee's challenge to the sufficiency of the evidence and to the adequacy of the jury instructions. We granted review, and subsequently limited the issues to whether the Act as amended by Proposition 83 violated McKee's constitutional rights under the due process, equal protection, and ex post facto clauses.

II. THE SVP ACT AND PROPOSITION 83

As originally enacted, an SVP was defined as "a person who has been convicted of a sexually violent offense against two or more victims for which he or she received a determinate sentence and who has a diagnosed mental disorder that makes the person a danger to the health and safety of others in that it is likely that he or she will engage in sexually violent criminal behavior." (Former § 6600, subd. (a).) A "sexually violent offense" included a Penal Code section 288 lewd act on a child under age 14. (Former § 6600, subd. (b); Hubbart, supra, 19 Cal.4th at p. 1145.) Under the Act, a person is "likely" to engage in sexually violent criminal behavior (i.e., reoffend) if he or she "presents a substantial danger, that is, a serious and well-founded risk, that he or she will commit such crimes if free in the community." ( People v. Superior Court (Ghilotti) (2002) 27 Cal.4th 888, 922 [ 119 Cal.Rptr.2d 1, 44 P.3d 949], italics omitted.)

The Act was "designed to ensure that the committed person does not `remain confined any longer than he suffers from a mental abnormality rendering him unable to control his dangerousness.' [Citation.]" ( Hubbart, supra, 19 Cal.4th at p. 1177.) The Act therefore provides "two ways a defendant can obtain review of his or her current mental condition to determine if civil confinement is still necessary. [First,] [s]ection 6608 permits a defendant to petition for conditional release to a community treatment program. . . . [Second,] [s]ection 6605 [requires] an annual review of a defendant's mental status that may lead to unconditional release." ( People v. Cheek (2001) 25 Cal.4th 894, 898 [ 108 Cal.Rptr.2d 181, 24 P.3d 1204], fn. omitted.)

On November 7, 2006, California voters passed Proposition 83, entitled "The Sexual Predator Punishment and Control Act: Jessica's Law" amending the Act effective November 8, 2006. Proposition 83 is a wide-ranging initiative that seeks to address the problems posed by sex offenders. It increases penalties for sex offenses, both by altering the definition of some sex offenses and by providing longer penalties for some offenses as well as modifying probation and parole provisions: it requires a GPS tracking device for felons subject to such registration for the remainder of their lives; it prohibits a registered sex offender from living within 2,000 feet of schools and parks; and it changes the SVP Act by reducing the number of sexually violent offenses that qualify an offender for SVP status from two to one. (See Voter Information Guide, Gen. Elec. (Nov. 7, 2006) analysis of Prop. 83 by Legis. Analyst, pp. 43-44.) Proposition 83 also changes an SVP commitment from a two-year term to an indefinite commitment. It is this latter provision with which this case is concerned and which will be described in more detail below.

Pursuant to Proposition 83, section 6604, which had prescribed a two-year term for SVP's, now provides in relevant part: "If the court or jury determines that the person is a sexually violent predator, the person shall be committed for an indeterminate term to the custody of the [DMH] for appropriate treatment and confinement. . . ." (Italics added.) Proposition 83 did not change section 6604's requirement that a person's initial commitment as an SVP be proved at trial beyond a reasonable doubt. Under Proposition 83, section 6605 continues to require current examinations of a committed SVP at least once every year. (§ 6605, subd. (a).) However, Proposition 83 added new provisions to section 6605 regarding the DMH's obligations: Pursuant to section 6605, subdivision (a), the DMH now files an annual report in conjunction with its examination of SVP's that "shall include consideration of whether the committed person currently meets the definition of a sexually violent predator and whether conditional release to a less restrictive alternative or an unconditional release is in the best interest of the person and conditions can be imposed that would adequately protect the community." Subdivision (b) now provides that "[i]f the [DMH] determines that either: (1) the person's condition has so changed that the person no longer meets the definition of a sexually violent predator, or (2) conditional release to a less restrictive alternative is in the best interest of the person and conditions can be imposed that adequately protect the community, the director shall authorize the person to petition the court for conditional release to a less restrictive alternative or for an unconditional discharge." (§ 6605, subd. (b).) If the state opposes the director's petition, then, as under the pre-Proposition 83 statute, it must prove beyond a reasonable doubt that the person still meets the definition of an SVP.

In the event the DMH does not authorize the committed person to file a petition for release pursuant to section 6605, the person nevertheless may file, as was the case with the pre-Proposition 83 Act, a petition for conditional release for one year and subsequent unconditional discharge pursuant to section 6608. (§ 6608, subd. (a).) Section 6608, subdivision (i), which was also unamended by the Act, provides: "In any hearing authorized by this section, the petitioner shall have the burden of proof by a preponderance of the evidence." (Italics added.) After a trial court denies a section 6608 petition, "the person may not file a new application until one year has elapsed from the date of the denial." (§ 6608, subd. (h).)

In short, under Proposition 83, an individual SVP's commitment term is indeterminate, rather than for a two-year term as in the previous version of the Act. An SVP can only be released conditionally or unconditionally if the DMH authorizes a petition for release and the state does not oppose it or fails to prove beyond a reasonable doubt that the individual still meets the definition of an SVP, or if the individual, petitioning the court on his own, is able to bear the burden of proving by a preponderance of the evidence that he is no longer an SVP. In other words, the method of petitioning the court for release and proving fitness to be released, which under the former Act had been the way an SVP could cut short his two-year commitment, now becomes the only means of being released from an indefinite commitment when the DMH does not support release.

III. DISCUSSION

McKee contends his indefinite involuntary commitment as an SVP under the Act violates his federal constitutional right to due process of law. There is no question that civil commitment itself is constitutional so long as it is accompanied by the appropriate constitutional protections. "States have in certain narrow circumstances provided for the forcible civil detainment of people who are unable to control their behavior and who thereby pose a danger to the public health and safety. [Citations.] . . . It . . . cannot be said that the involuntary civil confinement of a limited subclass of dangerous persons is contrary to our understanding of ordered liberty." ( Kansas v. Hendricks (1997) 521 U.S. 346, 357 [ 138 L.Ed.2d 501, 117 S.Ct. 2072] ( Hendricks).) In order to properly justify a civil commitment, "[a] finding of dangerousness, standing alone, is ordinarily not a sufficient ground upon which to justify indefinite involuntary commitment. We have sustained civil commitment statutes when they have coupled proof of dangerousness with the proof of some additional factor, such as a `mental illness' or `mental abnormality.' [Citations.]" ( Id. at p. 358.)

McKee contends that it is the fact that his commitment is now indefinite, and that it is his burden to show by a preponderance of the evidence that he is no longer an SVP, that violates his federal due process rights. In making this argument, he relies in large part on Addington v. Texas (1979) 441 U.S. 418 [ 60 L.Ed.2d 323, 99 S.Ct. 1804] ( Addington). In Addington, the court held unconstitutional a civil involuntary commitment statute that authorized an indefinite commitment when the state proved by a preponderance of the evidence that the individual was mentally incompetent. ( Id. at pp. 419-422.) As Addington explained: "The state has a legitimate interest under its parens patriae powers in providing care to its citizens who are unable because of emotional disorders to care for themselves; the state also has authority under its police power to protect the community from the dangerous tendencies of some who are mentally ill. Under the Texas Mental Health Code, however, the State has no interest in confining individuals involuntarily if they are not mentally ill or if they do not pose some danger to themselves or others." ( Addington, supra, 441 U.S. at p. 426.) The Addington court assessed the risk of improperly subjecting an individual to civil commitment: "At one time or another every person exhibits some abnormal behavior which might be perceived by some as symptomatic of a mental or emotional disorder, but which is in fact within a range of conduct that is generally acceptable. Obviously, such behavior is no basis for compelled treatment and surely none for confinement. However, there is the possible risk that a factfinder might decide to commit an individual based solely on a few isolated instances of unusual conduct. Loss of liberty calls for a showing that the individual suffers from something more serious than is demonstrated by idiosyncratic behavior. Increasing the burden of proof is one way to impress the factfinder with the importance of the decision and thereby perhaps to reduce the chances that inappropriate commitments will be ordered. [¶] The individual should not be asked to share equally with society the risk of error when the possible injury to the individual is significantly greater than any possible harm to the state." ( Id. at pp. 426-427.)

The Addington court therefore concluded that "the individual's interest in the outcome of a civil commitment proceeding is of such weight and gravity that due process requires the state to justify confinement by proof more substantial than a mere preponderance of the evidence." ( Addington, supra, 441 U.S. at p. 427.) It held that due process required proof by clear and convincing evidence at the appellant's initial civil commitment hearing. ( Id. at p. 433.)

McKee argues Addington requires the state to prove by at least clear and convincing evidence in not only the first commitment hearing but periodically at subsequent commitment hearings as well, and that therefore section 6608, subdivision (i) violates due process by imposing on the petitioner the burden of proving by a preponderance of the evidence that he is entitled to release. As explained below, the United States Supreme Court case law decided after Addington leads to the conclusion that the clear and convincing evidence standard does not apply to subsequent commitment proceedings for SVP's.

The primary case relied on by the People is Jones v. United States (1983) 463 U.S. 354 [ 77 L.Ed.2d 694, 103 S.Ct. 3043] ( Jones). Jones considered a District of Columbia statute that governed civil commitment of those who had been adjudged not guilty by reason of insanity of criminal charges (NGI's). Under that statutory scheme, a defendant was required to prove his affirmative defense of insanity by a preponderance of the evidence. ( Id. at p. 356, fn. 1.) After his acquittal by reason of insanity, another statute provided for his immediate commitment, with a hearing required within 50 days to determine whether he was eligible for release. At the hearing, he had "the burden of proving by a preponderance of the evidence that he [was] no longer mentally ill or dangerous. [Citation.]" ( Id. at p. 357.) If he did not meet that burden at the 50-day hearing, he was "entitled [by statute] to a judicial hearing every six months at which he may establish by a preponderance of the evidence that he is entitled to release. [Citation.]" ( Id. at p. 358, fn. omitted.)

The court rejected a due process challenge to the statute. Congress had determined "that a criminal defendant found not guilty by reason of insanity in the District of Columbia should be committed indefinitely to a mental institution for treatment and the protection of society. [Citations.]" ( Jones, supra, 463 U.S. at pp. 361-362.) An NGI determination "establishe[d] two facts: (i) the defendant committed an act that constitutes a criminal offense, and (ii) he committed the act because of mental illness." ( Id. at p. 363.) Jones stated: "Congress has determined that these findings constitute an adequate basis for hospitalizing the acquittee as a dangerous and mentally ill person. [Citations.] We cannot say that it was unreasonable and therefore unconstitutional for Congress to make this determination. [¶] The fact that a person has been found, beyond a reasonable doubt, to have committed a criminal act certainly indicates dangerousness. [Citation.] Indeed, this concrete evidence [of commission of a criminal act] generally may be at least as persuasive as any predictions about dangerousness that might be made in a civil-commitment proceeding." ( Id. at p. 364, fns. omitted.)

Distinguishing Addington, the court explained that in equating NGI commitment with the ordinary civil commitment at issue in Addington, "petitioner ignores important differences between the class of potential civil-commitment candidates and the class of insanity acquittees that justify differing standards of proof. The Addington Court expressed particular concern that members of the public could be confined on the basis of `some abnormal behavior which might be perceived by some as symptomatic of a mental or emotional disorder, but which is in fact within a range of conduct that is generally acceptable.' [Citations.] . . . But since automatic commitment under [the District of Columbia's NGI commitment statute] follows only if the acquittee himself advances insanity as a defense and proves that his criminal act was a product of his mental illness, there is good reason for diminished concern as to the risk of error. More important, the proof that he committed a criminal act as a result of mental illness eliminates the risk that he is being committed for mere `idiosyncratic behavior[.]' [Citation.]" ( Jones, supra, 463 U.S. at p. 367, fns. omitted.) Jones therefore concluded that "concerns critical to our decision in Addington are diminished or absent in the case of insanity acquittees. Accordingly, there is no reason for adopting the same standard of proof in both cases. . . . The preponderance of the evidence standard comports with due process for commitment of insanity acquittees." ( Jones, supra, 463 U.S. at pp. 367-368.)

Although McKee was not found not guilty by reason of insanity, he has been found beyond a reasonable doubt in his initial commitment to meet the definition of an SVP. That finding is, for present constitutional purposes, the functional equivalent of the NGI acquittal in Jones. As in Jones, McKee has already been found not only to have previously committed the requisite criminal acts but was found beyond a reasonable doubt to have "a diagnosed mental disorder that makes the person a danger to the health and safety of others in that it is likely that he or she will engage in sexually violent criminal behavior." (§ 6600, subd. (a).) Therefore, as in Jones, the danger recognized in Addington "that members of the public could be confined on the basis of `some abnormal behavior which might be perceived by some as symptomatic of a mental or emotional disorder, but which is in fact within a range of conduct that is generally acceptable'" or "for mere `idiosyncratic behavior'" ( Jones, supra, 463 U.S. at p. 367) is greatly diminished. Accordingly, as in Jones, the requirement that McKee, after his initial commitment, must prove by a preponderance of the evidence that he is no longer an SVP does not violate due process.

McKee seeks to distinguish Jones, arguing that in the latter case, an individual was entitled to a hearing every six months, whereas there is no such entitlement for the SVP after Proposition 83. In fact, section 6608, subdivision (h) permits an SVP to file a new petition for release as early as a year after the previous petition was denied. The statute does permit the court to deny a hearing if the petition is frivolous. Section 6608, subdivision (a) provides in pertinent part: "If a person has previously filed a petition for conditional release without the concurrence of the director and the court determined, either upon review of the petition or following a hearing, that the petition was frivolous or that the committed person's condition had not so changed that he or she would not be a danger to others in that it is not likely that he or she will engage in sexually violent criminal behavior if placed under supervision and treatment in the community, then the court shall deny the subsequent petition unless it contains facts upon which a court could find that the condition of the committed person had so changed that a hearing was warranted. Upon receipt of a first or subsequent petition from a committed person without the concurrence of the director, the court shall endeavor whenever possible to review the petition and determine if it is based upon frivolous grounds and, if so, shall deny the petition without a hearing." McKee contends that the court's discretion to deny a petition without a hearing as frivolous denies due process. We disagree. A frivolous petition is one that "indisputably has no merit." ( In re Marriage of Flaherty (1982) 31 Cal.3d 637, 650 [ 183 Cal.Rptr. 508, 646 P.2d 179] [defining frivolous appeals].) McKee cites no authority for the proposition that due process is violated by not granting such petitions a hearing. The fact that the statute gives the court the authority to deny such petitions does not, of itself, serve as an obstacle to the primary due process goal of ensuring that only those individuals who continue to meet SVP criteria will remain involuntarily committed.

McKee further contends that his lack of access to mental health experts to challenge his continuing commitment violates due process. As he points out, although section 6605, subdivision (d) mandates the appointment of experts when the DMH authorizes an indigent inmate to petition for release, section 6608, subdivision (a) merely provides that petitioner has the right to counsel, with no mention of experts, when he petitions without the DMH's approval.

McKee is correct that expert testimony is critical in an SVP commitment proceeding, in which the primary issue is not, as in a criminal trial, whether the individual committed certain acts, but rather involves a prediction about the individual's future behavior. If the state involuntarily commits someone on the basis of expert opinion about future dangerousness, places the burden on that person to disprove future dangerousness, and then makes it difficult for him to access his own expert because of his indigence to challenge his continuing commitment, that schema would indeed raise a serious due process concern.

We do not believe, however, that the statute needs to be interpreted in this narrow manner. It is true that section 6608, subdivision (a), unlike section 6605, subdivision (d), does not explicitly provide for experts when an SVP petitions the court for release. But section 6605, subdivision (a) states that in conjunction with the DMH's examination of an SVP's mental condition, which must occur "at least once every year," an SVP who is indigent may request and the court may appoint "a qualified expert or professional person to examine him or her." Although section 6605, subdivision (a) does not explicitly provide for the appointment of the expert in conjunction with a section 6608 petition, such appointment may be reasonably inferred. As is clear from the context, the annual examination authorized by section 6605, subdivision (a), occurs not solely or even primarily for the purpose of assessing the SVP's treatment needs, but mainly for determining whether involuntary commitment is still required, or whether the SVP has sufficiently changed as a result of treatment to be released. There is no indication that the Legislature that authorized these expert appointments on behalf of an indigent SVP believed that such experts should be disallowed from testifying at an SVP's section 6608 hearing, nor that an SVP's indigence should serve as an obstacle to such testimony. On the contrary, the statute appears to encourage state-funded qualified expert appointed for a petitioner's benefit so as to ensure that the commitment lasts no longer than is necessary.

We construe statutes when reasonable to avoid difficult constitutional issues. (See In re Smith (2008) 42 Cal.4th 1251, 1269 [ 73 Cal.Rptr.3d 469, 178 P.3d 446].) (9) After Proposition 83, it is still the case that an individual may not be held in civil commitment when he or she no longer meets the requisites of such commitment. An SVP may be held, as the United States Supreme Court stated under similar circumstances, "as long as he is both mentally ill and dangerous, but no longer." ( Foucha v. Louisiana (1992) 504 U.S. 71, 77 [ 118 L.Ed.2d 437, 112 S.Ct. 1780].) Given that the denial of access to expert opinion when an indigent individual petitions on his or her own to be released may pose a significant obstacle to ensuring that only those meeting SVP commitment criteria remain committed, we construe section 6608, subdivision (a), read in conjunction with section 6605, subdivision (a), to mandate appointment of an expert for an indigent SVP who petitions the court for release.

Construing the amended Act in the above manner, we conclude it does not violate the due process clause.

B. Ex Post Facto Claim

McKee also contends his indefinite commitment under the terms of Proposition 83 violated the federal constitutional prohibition against ex post facto laws because it is punitive and was applied to his conduct prior to its enactment. We disagree.

Article I, section 10 of the United States Constitution provides: "No state shall . . . pass any . . . ex post facto law. . . ." The ex post facto clause prohibits only those laws that "retroactively alter the definition of crimes or increase the punishment for criminal acts." ( Collins v. Youngblood (1990) 497 U.S. 37, 43 [ 111 L.Ed.2d 30, 110 S.Ct. 2715].)

In Hubbart, supra, 19 Cal.4th 1138, we made clear, in considering an ex post facto challenge to the pre-Proposition 83 version of the Act, that the Legislature had "disavowed any `punitive purpose[],' and declared its intent to establish `civil commitment' proceedings in order to provide `treatment' to mentally disordered individuals who cannot control sexually violent criminal behavior. [Citations.] The Legislature also made clear that, despite their criminal record, persons eligible for commitment and treatment as SVP's are to be viewed `not as criminals, but as sick persons.' [Citation.] Consistent with these remarks, the [Act] was placed in the Welfare and Institutions Code, surrounded on each side by other schemes concerned with the care and treatment of various mentally ill and disabled groups." ( Hubbart, at p. 1171.)

In concluding that our Act is not punitive, and therefore not within the scope of the ex post facto clause, we relied on the United States Supreme Court's similar conclusion in Hendricks with respect to Kansas's Sexually Violent Predator Act. As the court stated: "Far from any punitive objective, the confinement's duration is instead linked to the stated purposes of the commitment, namely, to hold the person until his mental abnormality no longer causes him to be a threat to others. [Citation.] If, at any time, the confined person is adjudged `safe to be at large,' he is statutorily entitled to immediate release. [Citation.]" ( Hendricks, supra, 521 U.S. at pp. 363-364.)

We therefore concluded the Act was not punitive because "[v]iewed as a whole, the SVPA is also designed to ensure that the committed person does not `remain confined any longer than he suffers from a mental abnormality rendering him unable to control his dangerousness.'" ( Hubbart, supra, 19 Cal.4th at p. 1177, quoting Hendricks, supra, 521 U.S. at p. 364.) It is true that, in elaborating on this statement, we pointed to the fact that "each period of commitment is strictly limited and cannot be extended unless the state files a new petition and again proves, beyond a reasonable doubt, that the person is dangerous and mentally impaired. (§ 6604.)" ( Hubbart, supra, 19 Cal.4th at p. 1177.) But nothing in Hubbart suggests that these requirements are indispensable to shielding the Act from an ex post facto challenge. In fact, the nonpunitive objectives of the Act — treatment for the individual committed and protection of the public — remain the same after Proposition 83. Moreover, under the Act after Proposition 83, as before, a person is committed only for as long as he meets the SVP criteria of mental abnormality and dangerousness. As such, the Proposition 83 amendments at issue here cannot be regarded to have changed the essentially nonpunitive purpose of the Act.

McKee also argues that Proposition 83, taken as a whole, including increased provisions regarding the punishment for those convicted of sexually related offenses, evinces a punitive purpose. Obviously, the portion of Proposition 83 that concerns increased punishment for sex offenses is punitive. But the fact that the amendments to the civil commitment statute are part of the same legislative enactment as amendments to the Penal Code does not render the former amendments punitive.

McKee also argues the seven-factor test articulated in Kennedy v. Mendoza-Martinez (1963) 372 U.S. 144, 168-169 [ 9 L.Ed.2d 644, 83 S.Ct. 554], applies to invalidate the Proposition 83 amendments at issue here. As the United States Supreme Court has explained, the Mendoza-Martinez factors provide "a useful framework. These factors, which migrated into our ex post facto case law from double jeopardy jurisprudence, have their earlier origins in cases under the Sixth and Eighth Amendments, as well as the Bill of Attainder and the Ex Post Facto Clauses. [Citation.] Because the Mendoza-Martinez factors are designed to apply in various constitutional contexts, we have said they are `neither exhaustive nor dispositive' [citations], but are `useful guideposts' [citation]. The factors most relevant to our analysis are whether, in its necessary operation, the regulatory scheme: has been regarded in our history and traditions as a punishment; imposes an affirmative disability or restraint; promotes the traditional aims of punishment; has a rational connection to a nonpunitive purpose; or is excessive with respect to this purpose." ( Smith v. Doe (2003) 538 U.S. 84, 97 [ 155 L.Ed.2d 164, 123 S.Ct. 1140].)

Although one of the five factors — affirmative restraints on liberty — is present here, as it is with all involuntary civil commitments, the other four factors go against McKee: (1) civil commitment has historically been imposed nonpunitively on those whose inability to control their behavior poses a danger to the public (see Hendricks, supra, 521 U.S. at p. 357); (2) as discussed above, the amendments to the SVP Act under consideration here do not alter the Act's nonpunitive purpose of treatment and public protection; (3) the civil commitment has a rational connection with those purposes; and (4) even with indefinite commitment and alterations in the burden and standard of proof, the commitment authorized by the Act is not excessive and is designed to last only as long as that person meets the definition of an SVP. We therefore conclude that the Proposition 83 amendments do not make the Act punitive and accordingly do not violate the ex post facto clause.

C. Equal Protection Clause

McKee contends his involuntary commitment as an SVP under the Act, as amended by Proposition 83 in 2006, violated his federal constitutional right to equal protection under the law because it treats SVP's significantly less favorably than those similarly situated individuals civilly committed under other statutes. We conclude his claim has some merit and will require remand for further proceedings.

In re Moye, supra, 22 Cal.3d 457 ( Moye) is highly relevant to assessing McKee's claim and will be discussed at length. Moye followed In re Franklin (1972) 7 Cal.3d 126 [ 101 Cal.Rptr. 553, 496 P.2d 465], in which the court held that a person who has been found not guilty by reason of insanity can be civilly committed for at least the maximum term of the underlying offense, and may only be released from commitment during that term if he proves by a preponderance of the evidence that he is no longer a danger to the health and safety of himself or others. ( Id. at p. 148.) As Moye summarized, we explained in Franklin "that by reason of the prior judicial determination of insanity, `persons acquitted by reason of insanity fall within a special class, thereby providing a rational basis for differences in the treatment afforded them. . . . [¶] . . . "The special interest which the public has acquired in the confinement and release of people in this exceptional class results from the fact that there has been a judicial determination that they have already endangered the public safety and their own as a result of their mental conditions as distinguished from people civilly committed because of only potential danger."'" ( Moye, supra, at pp. 462-463.) The question presented in Moye, however, was whether the commitment can continue under the same rules after that maximum term, requiring an individual to prove by a preponderance of evidence that he was no longer in need of such commitment. ( Moye, supra, 22 Cal.3d at p. 460.)

This court concluded that such extension of the commitment would violate equal protection. The court compared NGI commitment to commitment under the former Mentally Disordered Sex Offender (MDSO) Act, the forerunner of the SVP Act, although unlike SVP's, those committed under the MDSO Act were civilly committed in lieu of a prison term, rather than after that term. As we observed: MDSO's "comprise a class of individuals quite similar to those, such as petitioner, who have been acquitted of a criminal offense by reason of insanity. Both classes, for example, involve persons who initially have been found to have committed a criminal act, but whose mental condition warrants a period of confinement for treatment in a state institution, in lieu of criminal punishment." ( Moye, supra, 22 Cal.3d at p. 463.) Although MDSO's had been initially subject to indefinite commitment, our own decisions cast doubt on the validity of such commitments and "the Legislature has subsequently enacted new provisions which limit the duration of all MDSO commitments . . . [to] the `longest term of imprisonment which could have been imposed for the offense or offenses of which the defendant was convicted. . . .'" ( Id. at p. 464.) The Legislature then added former section 6316.2, which provided "for a special extended commitment of one year beyond the maximum term of imprisonment following jury trial if it is found that the patient suffers from a mental disorder and, as a result thereof, `is predisposed to the commission of sexual offenses to such a degree that he presents a serious threat of substantial harm to the health and safety of others.' (§ 6316.2, subd. (a)(2).) Additional one-year commitments were available, following similar annual hearings. ( Id., subd. (h).)" ( Moye, supra, 22 Cal.3d at p. 464.) The burden of proof in these hearings was on the People to prove the individual suffers from a mental disorder that predisposes him to commit sexual offenses that represented danger to the public. ( Ibid.)

Thus, comparing the NGI and MDSO statutory schemes, we stated: "The foregoing provisions demonstrate the marked differences between the statutory commitment and release procedures applicable to MDSOs on the one hand and persons committed under section 1026 on the other. Yet, as we have noted the preconditions to both commitments are similar: the initial commitment follows commission of a criminal act and is based upon a finding of a mental disorder which might present a danger to others. The MDSO can be confined for only a limited period, measured by the maximum term for the underlying offense, unless thereafter the People (or other committing authority) can establish grounds for an extended commitment. In contrast, persons in petitioner's class face indefinite, lifetime confinement unless they can prove that their sanity has been restored." ( Moye, supra, 22 Cal.3d at pp. 464-465.)

The Moye court then reviewed other California civil commitment statutes, including the Lanterman-Petris-Short Act (LPS Act; § 5000 et seq.), commitment for juvenile offenders, and those deemed incompetent to stand trial, each of which had rules for recommitment similar to the MDSO Act. The court concluded: "In summary, our research reveals that commitments under section 1026 represent the sole instance of a potential lifetime confinement, imposed without regard to the nature of the underlying offense or the maximum punishment prescribed for it, and without the additional protection of periodic review and recommitment hearings. Thus, disparity of treatment seems clearly to exist." ( Moye, supra, 22 Cal.3d at p. 465.)

The court then reasoned: "Because petitioner's personal liberty is at stake, the People concede that the applicable standard for measuring the validity of the statutory scheme now before us requires application of the strict scrutiny standard of equal protection analysis. Accordingly, the state must establish both that it has a `compelling interest' which justifies the challenged procedure and that the distinctions drawn by the procedure are necessary to further that interest. [Citation.] At the very least, persons similarly situated must receive like treatment under the law. [Citation.] As we have noted, by reason of their commission of a prior criminal act and the finding of a mental disorder justifying the initial commitment, persons committed as MDSOs are `similarly situated' with persons like petitioner." ( Moye, supra, 22 Cal.3d at pp. 465-466.)

The court then rejected the People's attempts to justify the differences in treatment between NGI and MDSO commitments. "The People suggest that MDSOs suffer from a more `limited' form of mental disorder (predisposition toward commission of sexual offenses) when compared with persons found to be insane under [Penal Code] section 1026. It seems quite clear, however, that both classes of persons present equally substantial risks of harm. By statutory definition, an MDSO is a person `who by reason of mental defect, disease, or disorder, is predisposed to the commission of sexual offenses to such a degree that he is dangerous to the health and safety of others.' (Welf. Inst. Code, § 6300, italics added.) Yet, despite their potential dangerousness, MDSOs must be released from confinement when the maximum term for their underlying offense has expired, unless the People can establish grounds for an extended commitment. ( Id., § 6316.2.) We believe that constitutional demands of equal protection require a similar shifting of the burden of proof in favor of persons acquitted as insane, in order to retain them in confinement beyond the maximum term prescribed for the offense they committed while insane." ( Moye, supra, 22 Cal.3d at p. 466.)

The court then concluded: "Specifically, we hold that principles of equal protection require . . . that persons committed to a state institution following acquittal of a criminal offense on the ground of their insanity cannot be retained in institutional confinement beyond the maximum term of punishment for the underlying offense of which, but for their insanity, they would have been convicted." ( Moye, supra, 22 Cal.3d at p. 467.) The court further concluded that "[t]o the extent practicable, and in the absence of further legislation on the subject, the procedure for the extended commitment of persons committed [under the NGI statute] should conform to the procedures specified in section 6316.2 of the Welfare and Institutions Code." ( Ibid.) Although the court did not mention it, section 6316.2, subdivision (e) then provided that MDSO's "shall be entitled to the rights guaranteed under the Federal and State Constitutions for criminal proceedings," which includes imposing on the People the burden of proving beyond a reasonable doubt that the individual continues to meet the definition of an MDSO. (Stats. 1977, ch. 164, § 3, pp. 634, 635; see In re Winship (1970) 397 U.S. 358, 364 [ 25 L.Ed.2d 368, 90 S.Ct. 1068] [proof beyond a reasonable doubt constitutionally required for criminal trials]; see also People v. Burnick (1975) 14 Cal.3d 306, 332 [ 121 Cal.Rptr. 488, 535 P.2d 352] [proof beyond a reasonable doubt required for MDSO's].)

Decisions by this court and the United States Supreme Court before and since Moye have used the equal protection clause to police civil commitment statutes to ensure that a particular group of civil committees is not unfairly or arbitrarily subjected to greater burdens. (See Baxstrom v. Herold (1966) 383 U.S. 107 [ 15 L.Ed.2d 620, 86 S.Ct 760] [when the state seeks to civilly commit a person after expiration of prison term, equal protection is violated when it does not afford a jury trial as for other civil committees]; Jackson v. Indiana (1972) 406 U.S. 715 [ 32 L.Ed.2d 435, 92 S.Ct. 1845] [the state may not indefinitely commit an individual deemed incompetent to stand trial, denying procedural protections afforded other civil committees]; Humphrey v. Cady (1972) 405 U.S. 504, 509 [ 31 L.Ed.2d 394, 92 S.Ct. 1048] [habeas corpus petitioner stated prima facie case that Wisconsin statute that civilly committed sex offenders violated equal protection by not providing jury trial available under the general civil commitment statute]; Foucha v. Louisiana, supra, 504 U.S. at pp. 84-85 [subjecting someone no longer insane to special civil commitment regime for insanity acquittees rather than to the general civil commitment statute violates equal protection]; In re Gary W. (1971) 5 Cal.3d 296, 307-308 [ 96 Cal.Rptr. 1, 486 P.2d 1201] [those committed by the former Youth Authority and subsequently civilly committed when they reach the age of 21 must pursuant to the equal protection clause be granted a jury trial afforded others civilly committed]; Conservatorship of Hofferber (1980) 28 Cal.3d 161, 178-179 [ 167 Cal.Rptr. 854, 616 P.2d 836] ( Hofferber) [equal protection requires the determination that a conservatorship under the LPS Act for an individual initially determined to be incompetent to stand trial be made by a jury beyond a reasonable doubt when it is claimed the individual has a dangerous mental disorder].)

Most recently in In re Smith, supra, 42 Cal.4th 1251 ( Smith), we concluded that a substantial equal protection issue was raised when a person imprisoned for a conviction overturned on appeal was treated as an SVP rather than forcing the state to meet the more stringent civil commitment requirements of the LPS Act. We therefore interpreted an ambiguous section of the SVP Act providing that SVP petitions are not to be dismissed because of "`good faith mistake of fact or law'" as not applying to such a person in order to avoid the equal protection issue. ( 42 Cal.4th at p. 1255.) In arriving at our conclusion, we summarized the relevant principles for applying equal protection analysis to civil commitment statutes: "(1) generally speaking, no individual or group when being civilly committed may be denied substantive or procedural protections that are provided to the population as a whole; (2) on the other hand, the Legislature may make reasonable distinctions between its civil commitment statutes based on a showing that the persons are not similarly situated, meaning that those who are reasonably determined to represent a greater danger may be treated differently from the general population; (3) in particular, those who are criminally convicted, and those indicted of criminal charges but incompetent to stand trial, may be distinguished, at least initially, from the general population for civil commitment purposes, because their criminal acts demonstrate that they potentially pose a greater danger to society than those not in the criminal justice system." ( Id. at pp. 1266-1267.)

Our statement of these principles in Smith followed an extensive discussion of Hofferber. In Hofferber, we found that those charged with criminal acts determined to be incompetent to stand trial could be subject to civil commitment under the LPS Act even though the state was not initially required to prove such persons were "gravely disabled" (§ 5008, subd. (h)(1)(A)) within the meaning of that act. ( Hofferber, supra, 28 Cal.3d at pp. 170-174.) "[The state] may adopt more than one procedure for isolating, treating, and restraining dangerous persons; and differences will be upheld if justified. [Citations.] Variation of the length and conditions of confinement, depending on degrees of danger reasonably perceived as to special classes of persons, is a valid exercise of state power." ( Id. at p. 172, italics added.) For this reason, we concluded, "some separate treatment of permanently incompetent criminal defendants formally charged with violent felonies is justified. Allegedly they have engaged in violence so critical that serious criminal charges were believed appropriate. Magistrates or grand juries have found substantial evidence that the alleged conduct actually was committed as alleged. Those determinations of probable cause establish strong grounds to believe that, by concrete acts, the incompetent defendants already have seriously imperiled public safety and thus are particularly dangerous." ( Id. at p. 173.)

In Smith, we concluded that the reasonably perceived greater danger of SVP's justifies their being treated differently from those subject to the LPS Act, the general civil commitment statute, who cannot be subject to long-term commitment based on psychiatric opinion alone. "Individuals in prison with felony convictions have yet to demonstrate their capacity or willingness to keep their conduct within the bounds of the law and to break old criminal habits, and the Legislature could legitimately conclude that such felons who have prior sexually violent offenses represent a particular danger to society that justifies a separate system of civil commitment." ( Smith, supra, 42 Cal.4th at p. 1268.) It is reasonable to suppose that those who have been adjudicated to have committed criminal acts represent a class that is on the whole more dangerous than those who have not — in other words, that such adjudication is a reasonable proxy for greater dangerousness.

With these principles in mind, we turn to McKee's equal protection claim. He contends that SVP's are treated less favorably than those similarly situated under the Mentally Disordered Offender (MDO) Act, Penal Code section 2960 et al., in violation of the equal protection clause. To evaluate this claim, we briefly review the MDO Act.

"As a condition of parole, a prisoner may be designated and civilly committed as an MDO for involuntary treatment of a `severe mental disorder' if certain conditions are met. ([Pen. Code,] §§ 2962, 2966; [citations].) Section 2962 provides that a prisoner is subject to the MDO Act if: `(a) The prisoner has a severe mental disorder that is not in remission or cannot be kept in remission without treatment'; `(b) The severe mental disorder was one of the causes of or was an aggravating factor in the commission of a crime for which the prisoner was sentenced to prison'; `(c) The prisoner has been in treatment for the severe mental disorder for 90 days or more within the year prior to the prisoner's parole or release'; `(d)' a special team of mental health professional[s] evaluated the prisoner and concluded that criteria (a), (b) and (c) above have been met, and that due to the severe mental disorder, the prisoner `represents a substantial danger of physical harm to others', `(e)' the prisoner received a determinate sentence for the crime referenced in subdivision (b), and the crime is one of the enumerated crimes in subdivision (e). (§ 2962, subds. (a)-(e).) If such are found to exist, the prisoner may request a de novo hearing before the Board of Parole Hearings. (§ 2966.) If the Board of Parole Hearings concludes that the criteria are met, the prisoner may request a jury trial in the superior court. ( Ibid.) `The standard of proof shall be beyond a reasonable doubt, and if the trial is by jury, the jury shall be unanimous in its verdict.' (§ 2966, subd. (b); [citation].)

"Before an MDO's current commitment period expires, the district attorney may petition to extend that commitment by one year. (§ 2970.) To do so, the medical director of the state hospital, the community program director, or the Director of Corrections first `shall submit' to the district attorney a written evaluation of the prisoner `[n]ot later than 180 days' before the prisoner's termination of parole or release. . . .'" ( People v. Allen (2007) 42 Cal.4th 91, 99 [ 64 Cal.Rptr.3d 124, 164 P.3d 557], fn. omitted.) An MDO's commitment may be extended by one year if it is once again established beyond a reasonable doubt by a unanimous jury verdict that the individual meets the definition of an MDO. (Pen. Code, § 2972, subds. (a) (c).)

As stated above, a prisoner is only eligible for MDO commitment if he or she has committed certain crimes of violence. These crimes are set forth in Penal Code section 2962, subdivision (e) and include voluntary manslaughter, kidnapping, carjacking, rape, forcible sodomy, armed robbery, arson, attempted murder, and other crimes in which the prisoner used force or violence or caused serious bodily injury.

In summary, SVP's under the amended Act are given indeterminate commitments and thereafter have the burden to prove they should be released (unless the DMH authorizes a petition for release). In contrast, an MDO is committed for one-year periods and thereafter has the right to be released unless the People prove beyond a reasonable doubt that he or she should be recommitted for another year. There is therefore no question that, after the initial commitment, an SVP is afforded different and less favorable procedural protections than an MDO.

As we have stated: "`The first prerequisite to a meritorious claim under the equal protection clause is a showing that the state has adopted a classification that affects two or more similarly situated groups in an unequal manner.' [Citations.] This initial inquiry is not whether persons are similarly situated for all purposes, but `whether they are similarly situated for purposes of the law challenged.'" ( Cooley v. Superior Court, supra, 29 Cal.4th at p. 253.) In other words, we ask at the threshold whether two classes that are different in some respects are sufficiently similar with respect to the laws in question to require the government to justify its differential treatment of these classes under those laws.

The Court of Appeal, in rejecting McKee's equal protection challenge, concluded that SVP's and MDO's are not similarly situated. "The classifications of an SVP and an MDO are different. An SVP is defined as `a person who has been convicted of a sexually violent offense against one or more victims and who has a diagnosed mental disorder that makes the person a danger to the health and safety of others in that it is likely that he or she will engage in sexually violent criminal behavior.' (§ 6600, subd. (a)(1).) In contrast, an MDO is generally defined as a person with a severe mental disorder that cannot be kept in remission without treatment and that was a cause or factor in the commission of a felony offense and, because of that severe mental disorder, represents a substantial danger of physical harm to others. (Pen. Code, § 2962, subds. (a)-(e); People v. Allen, supra, 42 Cal.4th at p. 99.) Therefore, the dangers posed by an SVP and an MDO are different. An SVP is civilly committed for treatment and confinement, in part, because of the danger posed that he or she will likely engage in sexually violent criminal behavior in the future. An MDO is civilly committed for treatment and confinement, in part, because of a substantial danger he or she will physically harm others in the future. Although both SVP's and MDO's have mental disorders, the dangers they pose (which provide the bases for their respective civil commitments) are different and therefore they are not similarly situated."

All that the above passage demonstrates is the incontrovertible point that SVP's and MDO's do not share identical characteristics. But the identification of the above differences does not explain why one class should bear a substantially greater burden in obtaining release from commitment than the other.

We conclude that MDO's and SVP's are similarly situated for our present purposes. As was stated in In re Calhoun (2004) 121 Cal.App.4th 1315 [ 18 Cal.Rptr.3d 315], in which the court struck down a policy that granted to SVP's a more restricted right to refuse antipsychotic medication than MDO's, both MDO's and SVP's "have been found, beyond a reasonable doubt, to suffer from mental disorders that render them dangerous to others. The dangerous finding requires only an assessment of future dangerousness. It does not require proof of a recent overt act. Both have been convicted of a serious or violent felony. At the end of their prison terms, both have been civilly committed to the Department of Mental Health for treatment of their disorders. Furthermore, the purpose of the MDO Act and the SVPA is the same: to protect the public from dangerous felony offenders with mental disorders and to provide mental health treatment for their disorders." ( Id. at pp. 1351-1352; accord, People v. Buffington (1999) 74 Cal.App.4th 1149, 1156 [ 88 Cal.Rptr.2d 696] ( Buffington) [concluding that SVP's and MDO's are similarly situated because "they are currently suffering from a mental disorder that renders them dangerous"]; People v. Gibson (1988) 204 Cal.App.3d 1425, 1436 [ 252 Cal.Rptr. 56] [an MDO is similarly situated to other adult persons involuntarily committed because "[o]ne purpose of all of these pertinent involuntary commitment schemes is the protection of the public from the dangerous mentally ill and their involuntary commitment for treatment"].) We agree that these common features make SVP's and MDO's similarly situated. Therefore, when the state makes the terms of commitment or recommitment substantially less favorable for one group than the other, the case law reviewed above teaches that it is required to give some justification for this differential treatment.

In other terms, imposing on one group an indefinite commitment and the burden of proving they should not be committed, when the other group is subject to short-term commitment renewable only if the People prove periodically that continuing commitment is justified beyond a reasonable doubt, raises a substantial equal protection question that calls for some justification by the People. As the United States Supreme Court has made clear, standards and burdens of proof represent societal determinations of who should bear the risk that a court's or jury's judgment will be in error. ( Addington, supra, 441 U.S. at pp. 426-427.) Standards and burdens of proof, like other due process protections afforded both criminal defendants and persons subject to involuntary commitment, also balance the individual's fundamental liberty interest in not being incarcerated or involuntarily confined with the state's compelling interest in protecting society from dangerous persons, in punishing criminal behavior in the case of criminal defendants, and in treating mental illness in the case of civil committees. Because MDO's and SVP's have the same interest at stake — the loss of liberty through involuntary civil commitment — it must be the case that when society varies the standard and burden of proof for SVP's in the manner in which Proposition 83 did, it does so because of the belief that the risks involved with erroneously freeing SVP's from their commitment are significantly greater than the risks involved with freeing MDO's. (See Moye, supra, 22 Cal.3d at pp. 465-467.) A substantial question is raised about the basis for this belief.

The People argue that the state has a wide latitude in classifying different types of civil commitments, citing People v. Wilkinson (2004) 33 Cal.4th 821, 838 [ 16 Cal.Rptr.3d 420, 94 P.3d 551], a criminal case. There is no question that the determination of punishment for various offenses inherently involves value and policy determinations left to the Legislature, or to the people acting in a legislative capacity, and penal classifications will be upheld unless they are irrational. (See People v. Hofsheier (2006) 37 Cal.4th 1185, 1201 [ 39 Cal.Rptr.3d 821, 129 P.3d 29].) But as discussed above, the MDO and SVP Acts are not penal statutes. The differentiation between MDO's and SVP's must be made with reference to the goals of the statutes, i.e. treatment of the mentally disordered or public protection. Therefore, as discussed above, the Legislature may make reasonable distinctions between its civil commitment statutes based on a showing "that those who are reasonably determined to represent a greater danger may be treated differently from the general population." ( Smith, supra, 42 Cal.4th at p. 1266.) A prior adjudication of criminal conduct is a reasonable proxy for greater danger to the public and may therefore serve as a basis for treating civil committees subject to such an adjudication differently from the general class of individuals subject to civil commitment. ( Ibid.; see also Hofferber, supra, 28 Cal.3d at pp. 171-173.) This differential treatment may result at least initially in imposing a greater burden of proof in order to be released from involuntary commitment. ( Moye, supra, 22 Cal.3d at pp. 462-463.) Here, however, both SVP's and MDO's have suffered prior felony convictions, and both have been determined by mental health experts to suffer from mental disorders that make them a continuing danger. Therefore, the reasons for differential treatment are not immediately obvious from the face of the two statutory schemes.

The People also rely on a passage from Buffington that attempts to justify the fact that MDO's receive treatment while in prison while SVP's do not. As that court stated: "Prisoners who suffer from conditions that may with treatment be kept in remission are the target of the MDO Act, whereas the SVPA covers prisoners whose conditions pose a risk of future sexually violent criminal behavior and who may never be completely treated. (Pen. Code, § 2962; Welf. Inst. Code, § 6606, subd. (b).) Given these contrasting backgrounds and expectations related to treatment, we cannot say the two groups are similarly situated in this respect for equal protection purposes." ( Buffington, supra, 74 Cal.App.4th at p. 1163.) People v. Hubbert (2001) 88 Cal.App.4th 1202, 1222 [ 106 Cal.Rptr.2d 490], provides a somewhat more specific rendering of the above: "[T]he MDO law targets persons with severe mental disorders that may be kept in remission with treatment (Pen. Code, § 2962, subd. (a)), whereas the SVP [Act] targets persons with mental disorders that may never be successfully treated. (Welf. Inst. Code, § 6606, subd. (b).)" The People then argue that "[g]iven these `contrasting backgrounds and expectations related to treatment,' [SVP's and MDO's] are not similarly situated for purposes of how long they should be confined and treated."

The truth of this assertion is unclear from the face of the statutes in question. The two statutes cited in Buffington and Hubbart are Penal Code section 2962 and Welfare and Institutions Code section 6606, subdivision (b). Penal Code section 2962, subdivision (a) states, in pertinent part that a prisoner may be classified as an MDO if he "has a severe mental disorder that is not in remission or cannot be kept in remission without treatment." (Italics added.) Welfare and Institutions Code section 6606, subdivision (b) provides: "Amenability to treatment is not required for a finding that any person is a person described in Section 6600, nor is it required for treatment of that person. Treatment does not mean that the treatment be successful or potentially successful, nor does it mean that the person must recognize his or her problem and willingly participate in the treatment program."

In other words, Penal Code section 2962, subdivision (a) includes two classes of MDO's: those whose mental disorders are simply found to be not in remission, and those whose mental disorders are found to be in remission only due to treatment. While the statute therefore contemplates that the latter class of MDO's will need and respond to treatment, it also includes in the former class those whose illnesses are not in remission and do not necessarily respond to treatment. There is therefore little difference in this respect between MDO's and SVP's; section 6606, subdivision (b) envisions that some SVP's will, and some will not, respond to treatment.

It is true that unlike the SVP commitment, an MDO commitment initially arises as a condition of parole and is only extended beyond parole if the individual's mental disorder is found not to be in remission. (See Pen. Code, §§ 2962, 2970.) This circumstance may arguably evince a legislative expectation that those initially classified as MDO's as a condition of parole may be more amenable to treatment than persons subject to SVP commitment immediately following a prison sentence. But it is not evident that those who are committed as MDO's after their parole term has expired, who have not yet demonstrated an amenability to treatment and have been determined to pose a continuing danger as a result of a mental disorder, differ materially from SVP's in terms of danger to society and need for continuing commitment.

The Court of Appeal below, in concluding that SVP's are more of a danger than MDO's, relied upon the legislative findings to Proposition 83: "As the California Supreme Court noted, the Act, on its original enactment, `narrowly target[ed] "a small but extremely dangerous group of sexually violent predators that have diagnosable mental disorders [who] can be identified while they are incarcerated."' ( Cooley v. Superior Court, supra, 29 Cal.4th at p. 253.) [¶] Thereafter, on passage of Proposition 83, the voters' information pamphlet for Proposition 83 noted: `Sex offenders have very high recidivism rates. According to a 1998 report by the U.S. Department of Justice, sex offenders are the least likely to be cured and the most likely to reoffend, and they prey on the most innocent members of our society. More than two-thirds of the victims of rape and sexual assault are under the age of 18. Sex offenders have a dramatically higher recidivism rate for their crimes than any other type of violent felon.' (See Voters Information Guide, Gen. Elec. (Nov. 7, 2006) text of Prop. 83, p. 127; Historical and Stat. Notes, 47C West's Ann. Pen. Code (2008) foll. § 209, p. 52.)"

But these assertions, written into the findings of Proposition 83 by those who drafted the initiative, are not the same as facts, and an allusion to an uncited United States Department of Justice study does not make them so. When a constitutional right, such as the right to liberty from involuntary confinement, is at stake, the usual judicial deference to legislative findings gives way to an exercise of independent judgment of the facts to ascertain whether the legislative body "`has drawn reasonable inferences based on substantial evidence.'" ( Professional Engineers v. Department of Transportation (1997) 15 Cal.4th 543, 569 [ 63 Cal.Rptr.2d 467, 936 P.2d 473], quoting Turner Broadcasting System, Inc. v. FCC (1994) 512 U.S. 622, 666 [ 129 L.Ed.2d 497, 114 S.Ct. 2445] (lead opn. of Kennedy, J.); see also Spiritual Psychic Science Church v. City of Azusa (1985) 39 Cal.3d 501, 514 [ 217 Cal.Rptr. 225, 703 P.2d 1119].) Thus, for example, where a constitutional right to privacy is at issue, evidence introduced at trial may call into question legislative factfinding. ( American Academy of Pediatrics v. Lungren (1997) 16 Cal.4th 307, 354-356 [ 66 Cal.Rptr.2d 210, 940 P.2d 797].) Therefore, the legislative findings recited in the ballot initiative do not by themselves justify the differential treatment of SVP's. Nor do these findings reference any comparisons between SVP's and MDO's.

Nor is it a response to McKee's equal protection challenge that the SVP commitment statute does not violate the due process clause, as discussed above. Due process and equal protection protect different constitutional interests: due process affords individuals a baseline of substantive and procedural rights, whereas equal protection safeguards against the arbitrary denial of benefits to a certain defined class of individuals, even when the due process clause does not require that such benefits be offered. (See, e.g., Califano v. Westcott (1979) 443 U.S. 76 [ 61 L.Ed.2d 382, 99 S.Ct. 2655] [rule that welfare benefits may be granted to families with dependent children when the father becomes unemployed but not the mother violates equal protection].) Here, as in Moye and related cases, when certain due process protections for those civilly committed are guaranteed by statute, even if not constitutionally required, the denial of those protections to one group must be reasonably justified in order to pass muster under the equal protection clause.

McKee argues that NGI's and SVP's are also similarly situated and that a comparison of the two commitment regimes raises similar equal protection problems as discussed above. His argument has merit. NGI's as discussed are those who have committed criminal acts but have been civilly committed rather than criminally penalized because of their severe mental disorder. Under the current statutory scheme they may not be in civil custody longer than the maximum state prison term to which they could have been sentenced for the underlying offense (Pen. Code, § 1026.5, subd. (a); People v. Crosswhite (2002) 101 Cal.App.4th 494 [124 Cal.Rptr.2d 3011) unless at the end of that period the district attorney extends the commitment for two years by proving in a jury trial beyond a reasonable doubt that the person presents a substantial danger of physical harm to others because of a mental disease, defect, or disorder. (Pen. Code, § 1026.5, subd. (b)(1); People v. Haynie (2004) 116 Cal.App.4th 1224, 1226 [ 11 Cal.Rptr.3d 1631; People v. Superior Court ( Blakely) 60 Cal.App.4th 202, 216 [70 Cal.Rptr.2d 3881.) We agree that, as with MDO's, the People have not yet carried their burden of justifying the differences between the SVP and NGI commitment statutes.

We do not conclude that the People could not meet its burden of showing the differential treatment of SVP's is justified. We merely conclude that it has not yet done so. Because neither the People nor the courts below properly understood this burden, the People will have an opportunity to make the appropriate showing on remand. It must be shown that, notwithstanding the similarities between SVP's and MDO's, the former as a class bear a substantially greater risk to society, and that therefore imposing on them a greater burden before they can be released from commitment is needed to protect society. This can be shown in a variety of ways. For example, it may be demonstrated that the inherent nature of the SVP's mental disorder makes recidivism as a class significantly more likely. Or it may be that SVP's pose a greater risk to a particularly vulnerable class of victims, such as children. Of course, this latter justification would not apply to SVP's who have no history of victimizing children. But in the present case, McKee's previous victims were children. Or the People may produce some other justification.

The concurring and dissenting opinion objects to any such remand, declaring that "[w]hether society should treat sex crimes and their perpetrators differently from those who commit other crimes, however, is a judgment call for society to make, not a `fact' for a judge to determine after an evidentiary hearing." (Conc. dis. opn. of Chin, J., post, at p. 1229.) In support of its position it cites a criminal case, Powell v. Texas (1968) 392 U.S. 514 [ 20 L.Ed.2d 1254, 88 S.Ct. 2145], in which the United States Supreme Court rejected a constitutional challenge to the criminalization of public intoxication. If we were considering a criminal statute and the legislative judgment about how to punish sex crimes, we would agree with the concurring and dissenting opinion. But as discussed above, the SVP Act is not a penal statute, and distinctions in the terms of civil commitment statutes that substantially disfavor a particular group are to be made "on the basis of degree of danger presented." ( Hofferber, supra, 28 Cal.3d at p. 173.) Nor does the fact that we have recognized that SVP's are "`extremely dangerous'" ( Cooley v. Superior Court, supra, 29 Cal.4th at p. 253) resolve the equal protection claim before us, as the concurring and dissenting opinion appears to conclude. Here SVP's are being compared to other classes of persons subject to civil commitment who also are being committed because of the substantial danger they pose to society.

We therefore remand this case to the trial court to determine whether the People, applying the equal protection principles articulated in Moye and related cases discussed in the present opinion, can demonstrate the constitutional justification for imposing on SVP's a greater burden than is imposed on MDO's and NGI's in order to obtain release from commitment. The trial court may, if appropriate, permit expert testimony.

In remanding the case, we make clear that different classes of individuals civilly committed need not be treated identically. In Hofferber, supra, 28 Cal.3d 161, even as we affirmed that fundamental distinctions between classes of individuals subject to civil commitment are subject to strict scrutiny ( id. at p. 171, fn. 8), we also acknowledged the government's legitimate capacity to make reasonable distinctions: "The state has compelling interests in public safety and in humane treatment of the mentally disturbed. [Citation.] It may adopt more than one procedure for isolating, treating, and restraining dangerous persons; and differences will be upheld if justified. [Citations.] Variation of the length and conditions of confinement, depending on degrees of danger reasonably perceived as to special classes of persons, is a valid exercise of state power." ( Id. at pp. 171-172, fn. omitted.) Moreover, we have recognized "the importance of deferring to the legislative branch in an area which is analytically nuanced and dependent upon medical science." ( Hubbart, supra, 19 Cal.4th at p. 1156.) But the government has not yet shown that the special treatment of SVP's is validly based on the degree of danger reasonably perceived as to that group, nor whether it arises from any medical or scientific evidence. On remand, the government will have an opportunity to justify Proposition 83's indefinite commitment provisions, at least as applied to McKee, and demonstrate that they are based on a reasonable perception of the unique dangers that SVP's pose rather than a special stigma that SVP's may bear in the eyes of California's electorate.

Moreover, we emphasize that mere disagreement among experts will not suffice to overturn the Proposition 83 amendments. The trial court must determine whether the legislative distinctions in classes of persons subject to civil commitment are reasonable and factually based — not whether they are incontrovertible or uncontroversial. The trial court is to determine not whether the statute is wise, but whether it is constitutional.

IV. DISPOSITION

George, C. J., Kennard, J., Werdegar, J., and Corrigan, J., concurred.